SPL is for two players (the mechanisms can support more than two players, but the piece distribution is designed for two).
The SPL board is square, tiled, 4x4.
Each player receives 16 tiles total. The tiles have a number on one side, and are blank (or uniform) on the other. The distribution of numbers is one 0, four 1s, four 2s, two 3s, two 4s, one 5, one 6, and one 7.
Players take turns alternating placing tiles face down on one of the 16 locations of the board. Tiles may be placed on top of other tiles, forming stacks. A player may also choose to play a tile face up — in this case, that player decides where the next player must place their tile (but only the location; the choice of tile and whether to place it face up or face down are left to the next player).
When all tiles have been placed, look at the sum of the numbers in each stack. If the total value is less than 8, the person with the most points in that stack wins. If the total value is greater than 8, the person with the least points wins. If the total value is equal to 8, or the two players are tied, the last player to play on the stack (the one with a piece nearest to the top) wins.
The player who wins the the most stacks, wins the game.
During the turn, before tile placement, a player may offer the other a trade. This can be either in the open (e.g. "I'll give you a 2 for a 4 or a 6"), or secret (both players choose a tile and give it to the other). The traded tiles count towards the player who placed them, not the player who originally owned them. This variant essentially requires a computer, because the markings on the tiles (to indicate the player) would have to change after trading.